1. Field of the Invention
The present invention relates generally to flow field analysis during wind tunnel testing and, more specifically, to a velocimeter capable of simultaneously measuring flow velocity and temperature at several points in supersonic and hypersonic flows using vaporizing particles.
2. Description of the Related Art
Laser velocimeters measure the velocity of small particles entrained in a fluid flow field. A transmission optical system divides a single laser beam into two equal path length beams and focuses these beams so they cross at a point within the flow field. Since laser light is monochromatic and coherent, a fringe pattern will be formed within the sample volume at the cross-over point. As a small particle passes through the fringe pattern, it scatters light whose intensity will oscillate as the particle passes through the alternating light and dark regions of the fringe pattern. A portion of this scattered light is collected by a receiver which directs the collected light to a photocathode surface of a photomultiplier. The photomultiplier converts the optical signal to an electrical signal, which is a collection of Poisson distributed photo electrons whose average occurrence rate is proportional to the instantaneous light intensity at the photocathode. As the intensity increases from the photon resolved regime, i.e., one photon per response time of the photomultiplier, the additional photon arrivals within the response time add voltage to the output signal. With sufficient photons, the signal approaches a Gaussian shaped signal burst containing the oscillation frequency. A measurement of the oscillation frequency is multiplied by the distance between adjacent fringes to yield the velocity of the particle.
Improvements have been made in basic laser diagnostic technology to allow measurement of the velocity and temperature of the flow at one or more points in the flow field. One such improvement is Laser Doppler Velocimetry (LDV), which measures velocity at a point as a function of time. For example, U.S. Pat. No. 4,919,536 (issued to Komine) describes a system which uses a laser doppler spectral image converter. A flow field is seeded with particles, as in conventional laser velocimeters, which are illuminated by a collimated, monochromatic laser light sheet. Doppler shifted scattering from particle motion is imaged by an optical system. An optical frequency-to-intensity converter is located at the image plane such that the transmitted image contains a simultaneous two-dimensional measurement of flow velocity along a direction determined by a laser beam and observer direction. These images are observed directly or through a TV-2-D array camera and monitor or processed through a computer system.
Particle Image Velocimeter (PIV) also measures velocity in a planar image at a single time, while Laser Induced Fluorescence (LIF) and Coherent Anti-Stokes Raman Spectrocopy (CARS) can measure temperature at one or more locations. For example, U.S. Pat. No. 5,002,389 (issued to Benser) describes a pulsed fluorescence velocimeter that determines dynamic parameters of a gas flow from detection of fluorescent re-radiation of excited molecules of the gas.
Other known measurement techniques include Raman Excitation and Laser Induced Electronic Fluorescence (RELIEF) and Laser Induced Photochromic/Fluorescence Anemometry (LIPFA) both of which can give the velocity for several points and along lines for several time increments.
Methods which require solid or liquid tracer particles, including LDV, PIV, and LIPFA, all have limitations at high speeds where the particles lag rapid changes in velocity and do not correctly represent the flow field. Moreover, LIF and CARS give temperature (as well as density) but not velocity. RELIEF, which gives velocity without solid or liquid tracer particles, is extremely complex to implement and is limited in the flow temperature range and gas composition that is useable.